Guard storyboards.js scene enqueues against read-modify-write clobbering

[atomantic]

Problem

server/services/pipeline/storyboards.js (~33-117, 126-191): enqueueStoryboardSceneVideo / enqueueStoryboardShotStartFrame read scenes at entry, enqueue a job, then write back the entire pre-read array via updateStage. Two near-simultaneous enqueues for different scenes on the same issue can clobber each other's freshly-written sceneVideoJobId/startFrameJobId (jobs keep running untracked). Marked UNCERTAIN in audit — verify updateStage's internal serialization first.

Decision

If updateStage doesn't already merge per-issue serially against fresh state, change these two paths to pass a targeted per-index patch or mutator function so the write merges against the freshest persisted scenes array.

Acceptance

• Concurrent enqueues for different scenes both retain their job ids (add a unit test simulating interleaved calls); no change to single-call behavior

[atomantic]

UNCERTAIN resolved: the race is real. updateStage serializes but does not merge.

updateStage(id, stage, patch) is updateStageWithLatest(id, stage, () => patch), which does queue on the per-series write tail (issueStages.js). But the three storyboard paths passed the entire pre-read scenes array as the patch — computed outside the lock — and the tail shallow-merges it over the freshest stage. So the second of two concurrent enqueues reverts the first scene's freshly-stamped job id; the losing media job runs to completion untracked, and the filename hook drops the render because the target slot no longer carries its job id.

Fixed in #3411 by passing a mutator through updateStageWithLatest instead, so the read-modify-write of scenes happens inside the write region and only the targeted index is replaced — mirroring persistComicPageSlot in comicPages.js. refineStoryboardScenePrompt got the same treatment beyond the issue's stated scope: identical bug, same file, and an LLM round-trip between its read and write makes it the widest window of the three.

Falsified against the pre-fix code — all four new interleaving tests fail with the clobber symptom (expected undefined to be 'job-scene-0') and pass after.